To gain insights into the forces which govern the evolution of transposable elements, we will screen for and examine sequences homologous to the transposable kanamycin resistance determinant Tn5 present in the genome of diverse bacterial isolates, and address the following questions: (i) What is the species distribution of sequences related to Tn5's component IS50 element and central kan(r) gene? (ii) With what other types of transposable elements has the Tn5-like Kan(r) gene become associated? (iii) What factors control the copy number per genome of an IS element? (iv) What governs its size, and the number and size of the proteins it encodes? (v) How do the sites at the termini of an IS element which are recognized by transposase, and the gene for transposase itself diverge? These questions will be addressed using colony and Southern blot DNA hybridizations, restriction endonuclease mapping, electrophoresis of transposable element encoded proteins, functional tests of transposition proficiency, DNA sequencing, and chemostat growth reconstructions.